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Full text of "Tests with insecticides for control of Lygus bugs on alfalfa in southern Arizona, 1949-1950"

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November 1951 E-831 

United States Department of Agriculture 

Agricultural Research Administration 

Bureau of Entomology and Plant Quarantine 



TESTS WITH INSECTICIDES FOR CONTROL OF LYGUS BUGS 
ON ALFALFA IN SOUTHERN ARIZONA, 1949-1950 

By E. E. Russell -i/ and O. L. Barnes, , 

Division of Cereal and Forage Insect Investigations — 



In field tests made in southern Arizona from 1945 to 1948 (E-788), 
1.25 pounds of DDT, 1 pound of chlordane, 1.5 pounds of toxaphene, or 
sufficient benzene hexachloride to give 0.25 pound of the gamma isomer 
per acre gave economic control of lygus bugs — on seed alfalfa throughout 
the fruiting period. The reduction of populations with each of these in- 
secticides averaged more than 86 percent. 

Further tests with these insecticides on seed alfalfa were made in 
1949 and 1950, and tests with parathion, lindane, aldrin, methoxychlor, 
and mixtures of piper onyl butoxide and pyrethrins on hay alfalfa in 1949. 
The results of these tests are summarized in this paper. 

Materials and Methods 

The insecticides were applied as dilute dusts or as emulsion sprays. 
Most of the dusts were factory-mixed, but certain concentrations for use 
on small plots were mixed with hand equipment. They were prepared by 
adding inert materials, principally pyrophyllite, to wettable powders or 
dust bases. The emulsions were prepared by adding water to factory- or 
laboratory-mixed concentrates. 

On seed alfalfa dusts and sprays were applied by airplane on fields 
ranging from 10 to 120 acres and from the ground with a low-pressure 
power sprayer or a multiple-nozzle power duster on plots of 0.4 to 2.5 
acres. On hay alfalfa they were applied with the same low-pressure or 
hand-operated equipment on 0.05- to 0.1 -acre plots. 

The dosages of dusts ranged from 18 to 40 pounds, and of sprays from 
4 to 10 gallons per acre. 



l[ Retired August 31, 1950. 

2/ E. L. Whipple and J. E. Christiansen assisted in these studies. 

3/ Lygus hesperus Knight, L. elisus Van Duzee, and L. pratensis 
oblineatus (Say). 



-2- 

All applications on seed alfalfa were made on the first seed crop, 
which was either the second or third alfalfa crop of the year. A single 
application or the first of two applications was usually made during the 
late -bud stage, but in a few tests it was made immediately before budding 
began or after a few flowers had opened. In some tests a second appli- 
cation was made 2 to 3 weeks after the first. Applications on hay alfalfa 
were made from May to September to different crops in various stages of 
growth. 

Lygus bug populations were sampled with a 15-inch insect-collecting 
net. In seed alfalfa counts were made before treatment, 3 and 7 days 
after treatment, and at weekly intervals thereafter until the crop was 
mature, usually 5 to 6 weeks after the initial treatment. In hay alfalfa 
counts were made before treatment, usually 3 or 4 days after treatment, 
and in some tests 1 and 6 days after treatment. Control was based on 
the average number of lygus bugs in sweepings on treated plots as com- 
pared with those on untreated plots. 

Insecticide residues on samples of air-dried alfalfa threshings from 
several of the plots treated in 1949 were determined by the Division of 
Insecticide Investigations of this Bureau. 

It was impossible to obtain accurate data on yields of seed in most of 
the tests carried out in 1949 and 1950. 

Tests in Seed Alfalfa 

Tests in 1949 . --In the Salt River Valley in 1949 toxaphene, chlordane, 
DDT, and mixtures of toxaphene and DDT were compared in applications 
made by airplane, and chlordane alone and with DDT in applications with 
ground equipment. Each dosage of insecticide or mixture of insecticides 
was replicated two to four times in a single treatment. Lygus populations 
before treatment ranged from 2 to 6 bugs per net stroke, and peak popu- 
lations on untreated plots ranged from 7 to 12 bugs. Results of the tests 
are shown in table 1. 

In emulsion sprays applied by airplane toxaphene at 3 pounds and at 
1.5 pounds plus 1.25 pounds of DDT per acre gave respective reductions 
of 95 and 89 percent for 6 weeks after treatment. Reductions with other 
treatments ranged from 32 to 75 percent for the same period. Chlordane 
and toxaphene were more effective as emulsions than as dusts. Of par- 
ticular importance in these tests was the fact that none of the dusts were 
highly effective for 42 days, which included the fruiting period of the alfalfa- 
seed crop. These dusts at the same dosages had given good control in 
1945-1948 (see E-788). 

In the tests with ground sprayers seed yields were higher on all 
treated than on untreated plots. 

Threshings of alfalfa were analyzed chemically for the presence of 
insecticide residues. Samples were taken at harvesttime from the plant 



-3- 

material discarded in the windrow behind a combine harvester. A sample 
was collected from each of 20 widely scattered points in a 10-acre plot 
and air-dried under shelter; its air-dry weight was 2 pounds. Results of 
the analyses are shown in table 2. The length of time from treatment to 
sampling ranged from 53 to 70 days, and the mean maximum temperatures 
from 99.6° to 104° F. Rainfall ranged from 0.59 to 0.68 inch in most tests. 
Residues ranged from to 13.2 p.p. m. of organic chlorine, or up to 19.3 
p. p.m. of the insecticide. The data on residues are not sufficient to 
permit any definite conclusions, but they do give some idea of the approx- 
imate amounts of residue that might be expected on seed alfalfa from 
insecticide treatments under similar conditions. Residues were lower 
from dust than from spray treatments at equal dosages of active ingredient. 

Tests in 1950 . --Emphasis in 1950 was given to tests with toxaphene. 
It was applied at different rates in emulsion sprays by airplane and with 
ground sprayers, and in dusts by ground dusters. DDT was applied as a 
dust alone and with sulfur, and as an emulsion with toxaphene. Chlordane 
was applied as a dust and as an emulsion. These tests were conducted 
in the Salt River and Buckeye Valleys. Lygus counts ranged from 1 to 7 
bugs per net stroke before treatment and reached peaks of 5 to 15 bugs 
per net stroke on untreated plots during the fruiting period of the alfalfa- 
seed crop. The results of these tests are summarized in table 3. 

The outstanding feature in the 1950 tests was the high degree of lygus 
control (98 percent) obtained over a 6-week period from a single appli- 
cation of toxaphene on large fields. Reductions were equally satisfactory 
when 2 or 3 pounds were applied as emulsions by airplane or when 1.8 
pounds were applied as a dust with ground equipment. Two applications 
of 1.5 pounds of toxaphene were required to give approximately the same 
control that was obtained with one application of 2 or 3 pounds. Control 
was excellent the first 2 weeks but then declined to 81 percent before the 
second application increased the control to 99 to 100 percent during the 
last 3 weeks of the fruiting period. One application of 1.5 pounds of 
toxaphene plus 0.75 pound of DDT per acre in an emulsion was only a 
little less effective than the emulsions of toxaphene alone. 

All the dust treatments on 2. 5 -acre plots were made during the early 
bud stage of alfalfa development, but had to be repeated 3 weeks later to 
control the lygus bugs throughout the fruiting period. Although the lygus 
reductions were 98 to 100 percent for the first 2 weeks, they had fallen 
to 76 to 87 percent a week later, before the second application was made. 
From the time of the second application until the end of the fruiting period, 
reductions were 94 to 100 percent. 

A DDT -sulfur dust applied with ground equipment on a 20 -acre plot 
gave a high reduction of the bugs for 6 weeks after the application. 



- 4- 

In tests on 0.4-acre plots with emulsions applied with a power sprayer 
a dosage of 3 pounds of toxaphene per acre was somewhat more effective 
than 1.5 pounds of toxaphene or 2 pounds of chlordane. 

Tests in Hay Alfalfa 

During the 1S49 season at Mesa, parathion, lindane, aldrin, methoxy- 
chlor, and mixtures of piperonyl butoxide and pyrethrins were tested 
against lygus bugs on alfalfa grown for hay. The insecticides were applied 
with ground equipment to plots 0.05 to 0.1 acre in size. Before treatment 
lygus populations ranged from 0.4 to 7.2 per net stroke. The results of 
these tests are shown in table 4. 

Lindane in emulsion sprays was the most consistently effective material 
tested. Lygus reductions were as great as 97 percent in hay alfalfa 1 day 
after application, but the control had decreased considerably by the sixth 
day. On similar alfalfa growth parathion dusts gave practically the same 
reductions as did lindane sprays. Control of the bugs with sprays con- 
taining 0.6 pound of aldrin per acre was less than that with parathion dusts 
or lindane sprays applied at the same rate. Aldrin at 0.4 pound, methoxy- 
chlor at 2.5 pounds, and various dosages of piperonyl butoxide plus 
pyrethrins were all less effective than the lindane or parathion treatments. 

Discussion 

These tests show that toxaphene is effective for the control of lygus 
bugs on seed alfalfa in southern Arizona. It should be applied at the rate 
of 3 pounds per acre, preferably in sprays, when the alfalfa is in the bud 
stage, a few days before the early blossoms begin to open. One applica- 
tion is usually sufficient, to give economic control of the bugs until the 
seed pods begin to mature and there is no further danger of injury to the 
crop by the bugs. If a second application is required, the dosage should 
be reduced to 1.5 pounds per acre. If treatment should be needed during 
the blooming period, the toxaphene should be applied early in the morning 
or late in the afternoon while bees are not active in the field. 

The dosages of toxaphene suggested for the control of lygus bugs are 
also effective against grasshoppers that are often numerous and cause 
severe damage to seed alfalfa. 

A few years ago it was thought that insect damage on alfalfa could 
best be reduced and maximum yields obtained by growing the seed crop 
as rapidly as possible with little spring or summer irrigation. However, 
since it has been found that infestations of lygus bugs in seed alfalfa can 
be economically controlled with insecticides, most of the growers in 
southern Arizona have adopted the practice of irrigating their crops 
enough to insure a heavy growth. This practice increases the capacity 
of the plants to produce flowers and seed, but it also lengthens the 
fruiting period, when the plants are most susceptible to lygus bug injury. 



5- 



Therefore, heavier dosages or more applications of insecticides are now 
needed to control the bugs than were previously required for sparser 
growth and a shorter fruiting period. 

Summary 

Field tests were conducted in 1949 and 1950 with insecticides for the 
control of lygus bugs on alfalfa in southern Arizona. On seed alfalfa a 
3-pound dosage per acre of toxaphene gave better results than did lower 
dosages of this insecticide, dosages of 1 to 2 pounds of chlordane or 
DDT, or of mixtures containing 1.25 pounds of DDT plus 1.5 pounds of 
toxaphene or 1 pound of chlordane. Insecticide residues on alfalfa 
threshings sampled 53 to 70 days after treatment ranged from to 19.3 
p. p.m. Residues from emulsion sprays were higher than those from 
dusts. 

More than 90 percent of the lygus bugs in hay alfalfa 10 to 20 inches 
high were destroyed with 0.5 or 0.6 pound of lindane or parathion per 
acre. Reductions with 0.5 pound of piperonyl butoxide plus 0.06 pound 
of pyrethrins, 0.4 to 0.6 pound of aldrin, or 2.5 pounds of methoxychlor 
per acre, were less than those with the lindane or parathion. 

Caution: - -Alfalfa threshings or forage treated with DDT, toxaphene, 
chlordane, aldrin, or lindane should not be fed to dairy animals, meat 
animals being finished for slaughter, or to poultry. Hay treated with 
parathion should not be cut for at least 14 days after the application. 



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